fix: react-native Xcode 12 compatibility (#1847)

Summary:
Latest Xcode 12 fails to build while without a module to depend on React-Core directly hence this change is necessary for all native modules on iOS. For more details please check: https://github.com/facebook/react-native/issues/29633#issuecomment-694187116

## Changelog

React Native Xcode 12 compatibility

Pull Request resolved: https://github.com/facebook/flipper/pull/1847

Test Plan: Use this branch to install with an app running on Xcode 12.

Reviewed By: mweststrate

Differential Revision: D25963723

Pulled By: priteshrnandgaonkar

fbshipit-source-id: a9642e242b5605f0f748f8f3b962af8455756ff4

react-native/react-native-flipper/react-native-flipper.podspec

@@ -26,5 +26,5 @@ Pod::Spec.new do |s|
   s.pod_target_xcconfig = { "HEADER_SEARCH_PATHS" => "\"${PODS_ROOT}/Headers/Public/FlipperKit\"" }
   s.requires_arc = true
   s.compiler_flags = compiler_flags
-  s.dependency "React"
+  s.dependency "React-Core"
 end